im not actually sure about this because im new and i havent tried it. but you can try and add the Horizontal and Vertical values together to produce a factor which the motors move.
RightMotorValue = Vertical - Horizontal;
LeftMotorValue = Vertical + Horizontal;
so when the joystick is all the way to the left, R = 0 - (-joyval), L = 0 + (-joyval) making it turn left.
the only problem with this method is when the joystick is on one of the corners you get double the joyval.